1. Field of the Invention
The present invention relates to an extreme ultra violet light source device, which is used as a light source of exposure equipment, for generating extreme ultra violet (EUV) light by applying a laser beam to a target.
2. Description of a Related Art
In recent years, photolithography has made rapid progress toward finer fabrication with finer semiconductor processes. In the next generation, microfabrication of 100 nm to 70 nm, and even microfabrication of 50 nm or less will be required. For example, in order to fulfill the requirement for microfabrication of 50 nm or less, the development of exposure equipment with a combination of an EUV light source of about 13 nm in wavelength and a reduced projection reflective optics is expected.
There are three kinds of light which are used as an EUV light source: an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source device”, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has the advantages that extremely high intensity near black body radiation can be obtained because plasma density can be considerably made larger, light emission of only the necessary waveband can be performed by selecting the target material, and an extremely large collection solid angle of 2π steradian can be ensured because it is a point source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP light source is thought to be predominant as a light source for EUV lithography requiring power of several tens of watts.
FIG. 22 is a diagram for explanation of a principle of generating EUV light in the LPP system. An EUV light source device shown in FIG. 22 includes a laser oscillator 901, collector optics 902 such as a condenser lens and so on, a target supply unit 903, a target nozzle 904, and an EUV collector mirror 905. The laser oscillator 901 is a laser light source that pulse-oscillates to generate a laser beam for exciting a target material. The condenser lens 902 condenses the laser beam outputted from the laser oscillator 901 to a predetermined position. Further, the target supply unit 903 supplies the target material to the target nozzle 904 and injects the supplied target material to the predetermined position.
When the laser beam is applied to the target material injected from the target nozzle 904, the target material is excited and plasma is generated, and various wavelength components are radiated from the plasma.
The EUV collector mirror 905 has a concave reflection surface that reflects and collects the light radiated from the plasma. A film in which molybdenum and silicon are alternately stacked (Mo/Si multilayered film), for example, is formed on the reflection surface for selective reflection of a predetermined wavelength component (e.g., near 13.5 nm). Thereby, the predetermined wavelength component radiated from the plasma is outputted to an exposure tool or the like as output EUV light.
In the LPP type EUV light source device, there is a problem of the influence by charged particles such as fast ions emitted from plasma. This is because the EUV collector mirror 905 is located relatively near the plasma emission point (the position where the laser beam is applied to the target material), and thus, the fast ions and so on collide with the EUV collector mirror 905 and the reflection surface of the mirror (Mo/Si multilayered film) is sputtered and damaged. Here, in order to improve the EUV light generation efficiency, it is necessary to keep the reflectance of the EUV collector mirror 905 high. For this purpose, high flatness is required for the reflection surface of the EUV collector mirror 905, and the mirror becomes very expensive. Accordingly, longer life of the EUV collector mirror 905 is also desired so as to reduce operation costs of the exposure system including the EUV light source device, to reduce maintenance time, and so on.
As a related technology, U.S. Pat. No. 6,987,279 B2 discloses a light source device including a target supply unit that supplies a material as a target, a laser unit that generates plasma by applying a laser beam to the target, collector optics that collect and output extreme ultra violet light emitted from the plasma, and magnetic field generating means that generates a magnetic field within the collector optics for trapping charged particles emitted from the plasma when electric current is supplied (page 1, FIG. 1). In the light source device, ions generated from the plasma are trapped near the plasma by forming a mirror magnetic field by using electromagnets of Helmholtz type (column 6, FIG. 4). Thereby, the damage on the EUV collector mirror due to so-called debris such as ions is prevented.
Further, according to U.S. Pat. No. 6,987,279 B2, in order to efficiently eject ions and so on from the vicinity of the plasma and the collector mirror to reduce the concentration of the residual target gas (ions and neutralized atoms of the target material) near the plasma, the magnetic field is formed such that the magnetic flux density on the opposite side of the collector mirror becomes lower (columns 7-8, FIGS. 6A-7). Because of the action of the magnetic field, the ions and so on are guided in the direction of the lower magnetic flux density, that is, in the direction opposite to the collector mirror.
However, even when the ions, etc. are led out of the magnetic field in such a manner, the ions, etc. still need to be efficiently ejected out of the chamber. Otherwise, the concentration of the residual target gas (ions and neutralized atoms of the target material) within the chamber will rise. Since the target gas absorbs the EUV light radiated from the plasma, a problem is caused that the available EUV light decreases as the concentration rises. Therefore, it is necessary to locate a mechanism for efficiently ejecting the target gas out of the chamber (e.g., an ejection opening having a large diameter) in an appropriate position in addition to the configuration shown in FIGS. 6A and 7 of U.S. Pat. No. 6,987,279 B2.
In the case of providing a mechanism for ejecting ions, etc. in the device shown in FIGS. 6A and 7 of U.S. Pat. No. 6,987,279 B2, the following problem arises. In a general EUV light source, a filter for purifying the spectrum of EUV light, a coupling mechanism to an exposure tool, and so on are provided at the side opposite to the EUV collector mirror (in the traveling direction of the reflected EUV light). Therefore, in consideration of the interference with the filter, the coupling mechanism and so on, it is difficult to provide the mechanism for ejecting ions, etc. at the side opposite to the collector mirror. On the other hand, in the case where the position of the ejection mechanism, especially the ejection opening to be formed in the chamber, is inappropriate, the ejection speed of ions, etc. becomes lower and the concentration of ions, etc. rises within the chamber. Specifically, it is considered that such a tendency becomes stronger in the case where EUV light is generated by highly repeated operation.